1. Field of the Invention
This invention concerns a method of heating a drying furnace for use in hot air-circulating type drying of a coating, in which a hot air is supplied under circulation into a furnace for thereby heating and drying coated articles.
2. Description of the Prior Art
Drying furnaces for use in the coating drying mentioned above are generally classified into a direct heating type drying furnace adapted to supply combustion gases produced upon combustion of heavy oil, kerosene, city gas, liquefied propane gas and like other fuels by a burner, as they are, to the inside of the furnace, under circulation, and an indirect heating type (heat exchange type) drying furnace adapted to supply combustion gases or heat medium heated by the combustion gases from a burner to a heat exchanger for heating air and to supply the heated air, under circulation, to the inside of the furnace.
The direct heating type drying furnace is advantageous because of its extremely high heat efficiency and thus its capablility of rapidly increasing the temperature in the furnace, since the combustion gases from the burner are introduced, as they are, to the inside of the furnace. However, it has a drawback that undesired effects are caused due to impurities or the like in the quality of the coating films on the coated articles. Particularly, since combustion gases from a burner using petroleum type fuels contain a great amount of sulfur, they remarkably impair the quality of the coating films on the coated articles.
In view of the above, relatively clean fuel containing less impurity such as sulfur, for example, city gas or liquefied propane gas has generally been used in the case of a baking-finish furnace for drying a top coating (finish drying) in an automobile coating that demands an extremely high product quality.
However, as the result of experiment and study, the present inventors have found that, even when relatively clean fuel is used in a direct heating type of furnace for drying the finish on a car body, since the car body is heated to a high temperature of from about 120.degree. to 200.degree. C. about (usually from 150.degree. to 160.degree. C.), for example, in a drying furnace conducting final baking of the coating on the car body, organic solvents such as thinner and toluene, paint resins and amine compounds such as curing agent contained in the paint are evaporated in a great amount from the coating film. Then, they are thermally decomposed in direct contact with the flame of the burner and reacted with chemically active radicals formed in the combustion products such as water and nitrogen oxides (NOx) to produce tar-like substances mainly comprising deposits of low molecular weight resins, which adhere to the surface of the car body causing undesirable phenomena such as yellowing and inter-layer defoliation of the coating films.
Accordingly, in order to prevent these drawbacks inherent in the direct heating type drying furnace, it is necessary to use a burner with a high air/fuel ratio or to decrease the concentration of evaporated organic solvents or the like that lead to the formation of the tar-like substances, by increasing the amount of fresh air supplied to the inside and increasing the amount of contaminated air discharged from the inside thereof. However, this significantly increases the running cost.
On the other hand, the indirect heating type drying furnace is advantageous in that the coated articles can be isolated completely from the dust, impurities and the like produced from the combustion of the burner. Thus, the organic solvents, if evaporated from the coating film on the coated articles, are free from contact with the frame of the burner and thus produce no tar-like substances. Although this method can be considered suitable for the baking-finish of the car body or the like in this regard, the temperature increase rate is extremely low thereby requiring an extremely longer period of time for pre-heating the inside of the furnace as compared with the direct heating type drying furnace. Particularly, it is required for the drying furnace disposed in the continuous coating line of an automobile factory that the temperature can be increased to a predetermined level in a time as short as possible so as not to impair the productivity of the coating line, and this is considered to be an extremely serious drawback.